ci(testrunner): rebuild 3.15-dev against current CPython

Bump pyenv v2.6.31 to v2.8.4 so a docker/** push to main rebuilds the
testrunner image. The pinned image still has 3.15.0b3+dev (Jun 30),
which rejects rc1-built cp315 wheels.

Author: vlad-scherbich

docker/Dockerfile

@@ -119,8 +119,11 @@ COPY .python-version /home/bits/
 # Allow running datadog-ci in CI with npx
 RUN npm install -g @datadog/datadog-ci
 
-# Install pyenv and necessary Python versions
-RUN git clone --depth 1 --branch "v2.6.31" https://github.com/pyenv/pyenv "${PYENV_ROOT}" \
+# Install pyenv and necessary Python versions.
+# 3.15-dev is a rolling CPython 3.15 branch clone. Bumping pyenv retriggers
+# .github/workflows/testrunner.yml so the image is not stuck on 3.15.0b3+dev
+# (Jun 30 2026); rc1-built cp315 wheels fail to import on that interpreter.
+RUN git clone --depth 1 --branch "v2.8.4" https://github.com/pyenv/pyenv "${PYENV_ROOT}" \
   && pyenv local | xargs -L 1 pyenv install \
   && cd -